1. Field of the Invention
The present invention relates to deposition techniques for forming metal regions on semiconductor substrates, and more particularly, to the selective deposition of refractory metals on a refractory metal nitride and/or silicide barrier layer and the semiconductor devices formed thereby.
2. Description of the Prior Art
The continued miniaturization of integrated circuits has brought about an increasing need to reduce the resistance in the source-drain-gate and contact metallurgy. In recent years, much effort has been focused on the use of metal silicides to fulfill this need. However, as device dimensions become even smaller, both vertically and horizontally, silicides lose their attractiveness. The intrinsic resistivity of the silicides is high compared to metals, while the formation of self-aligned silicides consumes silicon in proportion to the thickness of the silicide that is formed. This consumption usually leads to junction leakage which is intolerable. U.S. Pat. 4,701,349 is directed to a method for depositing a self-aligned interconnect comprised of a layer of titanium nitride and a layer of titanium silicide. The sheet resistance of the titanium nitride layer is on the order of 15 ohm/sq.
Refractory metals have been investigated as possible alternatives to silicides. The low resistances and relatively high temperature stability makes the refractory metals attractive. In addition, the recent development of selective chemical vapor deposition (CVD) processes, have made tungsten and molybdenum prime candidates to replace silicides. In accordance with the CVD technique, tungsten (W) is deposited on the surface areas by placing the substrate in a CVD reactor and heating the substrate. Tungsten hexafluoride (WF.sub.6) and an inert carrier gas such as argon (Ar) or nitrogen (N.sub.2) are then fed into the reactor and the tungsten hexafluoride will react with the silicon in accordance with the following: EQU 2WF.sub.6 +3Si.fwdarw.2W+3SiF.sub.4
The deposition of tungsten will stop and in order to deposit additional material, hydrogen (H.sub.2) is added to the tungsten hexafluoride and carrier gas. The tungsten hexafluoride will react with the hydrogen to deposit the desired additional tungsten in accordance with the following: EQU WF.sub.6 +3H.sub.2 .fwdarw.W+6HF
The use of the above hydrogen reduction process to selectively deposit tungsten for VLSI applications has been limited by problems inherent in the deposition process. The problems include unpredictable, deep tungsten penetration into silicon regions (worm holes) due to attack of HF liberated in the reaction. In addition, encoachment problems occur due to the tungsten penetrating along nearby silicon dioxide/silicon interfaces. Furthermore, problems result from the poor adhesion of the tungsten to silicon.
Kotani, et al., IEDM (1987 IEEE) disclose a CVD process for the selective deposition of tungsten utilizing silane (SiH.sub.4) reduction. Kotani, et al. use a low temperature and pressure CVD process that results in the formation of undesirable silicide peaks which increase the resistivity of the tungsten layer to about 20 .mu.ohm-cm.
The use of barrier layers to enhance adhesion and lower contact resistance for tungsten films has been recently demonstrated by Brodsky, et al., IBM Tech. Bull., February, 1986. A titanium nitride layer was deposited by sputtering prior to the tungsten deposition by sputtering, electron beam evaporation or low pressure CVD. However, adhesion and resistance difficulties have been encountered in the deposition of selective tungsten on these barriers.